Protective coating



Patented Jan. 2, 1945 PROTECTIVE COATING Eldon D. Bayley, J r., Springfield, Ohio, assignor to The William Bayley Company,

Springfield.

Ohio, a corporation of Ohio No Drawing. Application June 22, 1939. Serial No. 280,652

1 Claim.

This invention relates to a process of providing a protective coating for a metallic surface and more particularly to a rust-resisting coating.

It has been well known in the art that a metal-- 110 surface, and particularly a ferrous surface, may be protected by a coating of metallic phosphates, which coating may be precipitated from a phosphoric acid solution saturated with iron phosphate, zinc phosphate, phate, copper phosphate, manganese dihydrogen phosphate and possibly other phosphates, so as to form a saturated solution. To this solution may be added an accelerator such as a suitable copper salt, as for example, copper nitrate. The body to be-coated, according to past practice, has first had the surface thoroughly cleansed and then submerged in the weak solution of phosphoric acid. saturated with metallic phosphates. The reaction of the weak phosphoric acid with the exposed iron surface seems to'neutralize the acid in the immediate vicinity of the body that is submerged. thereby causing this neutralized film to become super-saturated with the metallic phosphates, resulting in metallic phosphates being precipitated from the solution upon the surface of the metal. This takes place over a long period of time, until finally. the metallic phosphates provide an insulating coating impenetrable by the weak solution of phosphoric acid. It may take three-quarters of an hour to an hour or more to produce this coating. It has been found that this process results in a surface coating for a ferrous body that is rust-resisting.

An object of this invention is to coat the surface of a metallic article with a coating of metallic phosphates applied by a process including the use of an accelerating coating that neutralizes or reacts with a weak acid solution saturated with metallic phosphate thereby rapidl depositing the metallic phosphates upon the metallic surface.

Another object of this invention is to deposit a metallic oxide coating upon the surface of an article to be treated. which oxide coating neutralizes a weak acid solution saturated with metallic phosphates so as to cause metallic phosphates to be deposited upon the surface of the article.

Another object of this invention is to form a ferrosoferric oxide layer upon the surface of the iron body to be protected and then dip the article 5 to be protected into a weak acid solution saturated with metallic phosphates.

Another object of this invention is to provide a protective surface for a metallic article, includmanganese phosing the process of cleaning the surface of the article by placing it in fused sodium hydroxide and passing a current through the solution, using the article as the cathode, reversing the flow of the current, using the article as an anode, resulting in a black coating being formed upon the surface of the article, rinsing the article, then dipping the article into a saturated solution of weak phosphoric acid and metallic phosphates.

.Another object of this invention is to deposit a coating upon a surface to be coated, which coating is deposited from a saturated solution in which the article to be coated is dipped.

Another object of this invention is to provide a metallic surface with a coating that is easily applied. efiicient and dependable.

Another object of this invention is to accelerate the precipitation of metallic salts upon a metallic surface by the use of a reagent causing supersaturation.

Other objects and advantages reside in the proportion of the ingredients, the combination thereof and the mode of combining, a will become more apparent from the following description. 1

Due to the nature of the invention no drawings are deemed necessary.

The process of providing a protective coating by my invention includes broadly three steps: (1) the cleaning of the surface to'be treated, 2) the formation or application of an auxiliary coating and 3) the application of the protective coating.

The cleaning process The metallic surface may be cleansed by any suitable process which removesall rust, all corrosion. all coatings including scale, grease, oil and other foreign matter. lustrafiion, this process may be exemplified by the use of sodium hydroxide that has been heated to the melting point, assisted b a suitable electric current. More specifically, this process includes the steps of heating sodium hydroxide to a temperature of 800" F. or higher, the higher the better. An electrode is placed in a bath of this fused sodium hydroxide, which electrode is used as the anode. The body to be treated is then dipped into the bath and utilized as the cathode. This results in anions. that, is, hydroxyl. traveling towards the anode and kationsor the sodium ions. advancing towards the cathode. The sodium ions are nascent and have a great affinity for oxygen, thereby readily unitin with the oxygen found in any oxides that may be on the metallic surface. As this process is primarily used in connection with the surface treatment of ferrous bodies, the

For the purpose of il-.

sodium ions unite with the oxygen of the iron oxides in whatever forms these oxides may be.

' resulting in sodium oxide or oxides being formed.

probably including monoxide. The hydroxyl ions form water H and give off the excess oxygen. The H2O molecules, upon coming in contact with the sodium oxide molecules, form sodium hydroxide molecules. This process results in the removal of scale and rust very rapidly, re sulting in a clean metallic surface being exposed upon the body. The sodium hydroxide unites with the greases and oils on the surface of the article, so as to form soap. Other foreign matter, especially organic matter, appearing on the surface of the artcile to be cleaned, upon being subjected to the intense heat of the melted so dium hydroxide, is literally burned from the surface. 1

The sodium hydroxide is not lost or used up in carrying out the process, excepting the small quantity that may unite with the oils and greases to form soap and the losses caused by carrying sodium on the surface of the article. As already stated, the sodium oxides, formed by the chemical reaction with the oxygen of the metallic oxides on the surface of the article to be coated, eventually unite with H2O molecules formed from the hydroxy1-ions,'so as to be'reconverted into sodium hydroxide.

The resulting metallic surface has not been.

pitted or mutilated in any manner by/the cleaning process. The sodium ions being metallic, do not react with metals. Upon examiningthe cleansed irregular contour of the surface permits paint or the succeeding coating to firmly adhere to the metallic surface.

The cleansed metallic surface, especially if the metal erodes or corrodes easily, is not very satisfactory unless it is protected with a suitable coating. Various types of, coatings may be used. Metallic phosphates, upon being properly deposited upon the metallic article,'give satisfactory results as protective coatings in certain industries. These may be applied by several well known processes, some of which are extensively used in the industries, such as the Parkerizing process, the

Bonderizing process or the Cozlettizing process.

Depositing metallic phosphates upon the surface of a clean ferrous body either're'quires too much time or the coating is too thin, that is, to produce a satisfactory coating it may take an hour or more to properly coat the article. If the time is reduced, the quality of the protective coating suffers.

The black coating formed on the surface of the metal may be referred to as a temporary, displaceable, activating coating. It is believed to be temporary in that it is displaced by the metallic salts. Being it is displaced by the metallic salts. it may be referred to as displaceable. Due to the fact that this temporary, displaceable coating causes the metallic salts to be precipitated far more rapidly from the saturated solution, it is referred to asyactivatingyin that it accelerates the process. The explanation made as to this temporary, displaceable, activating coating is of-- fered as a plausible explanation, rather than a definite statement of fact.

' as iron or nickel oxide.

Formation or application of auxiliary coating saturated film overlying the body to be coated,

thereby causing the metallic salts to be precipitated very rapidly upon the surface of the body.

Various accelerating reagents may be used, such For the purpose of illus tration, the reagent for. accelerating the precipitation of the metallic salts may be produced by reversing the flow of the current through the cleansing solution.

. results in a black coating forming'upon the iron surface under a microscopait appears rough and irregular, due to irregularities in the oxides, that surface} which coating is believed to be ferrosoferric oxide F8304, or it may be a mixture of various oxides.

By reversing the flow of the current, the electiode that was the former cathode becomes the anode and. the electrode that wasv the former anode becomes the cathode. This reverses the g flow of the ions in the solution. The hydroxyl ions flow toward the ferrous article andaccumulate on the surface. As the sodium oxides come in contact with the hydroxyl ions on the'surface of the ferrous article, the nascent oxygen unites or oxidizes the surface of the article. Due to the nascent condition of the oxygen, this oxidation takes place very rapidly. Fifteen (15) seconds to one minute will produce a very satisfactory black coating, which functions as a reagent in depositing the metallic salts upon the surface of the article.

Due to the previous reduction of the oxides on the surface of the article, it is believed that the surface of the metal may consist of a porous network including nascent metal that readily unites with any free, oxygen and .particularly nascent oxygen. Upon removal from the sodium hydroxide bath, the article is then dipped into hot water having a small quantity'of sodium hydroxide added thereto, so as to rinse the surface of the article to remove caustic soda. After rinsing the article in this hot Water having added thereto a small quantity of sodium hydroxide, the surface is coated with a weak alkaline solution, which functions as a temporary protective coating, in that the article having the black coating covered with the weak alkaline temporary protective coating does not oxidize very readily. Thereby, the temporary alkaline coating reduces shop oxidation. Immediately before the article is to be dipped into the acid solution having metallic salts therein, the article is again rinsed, so as to remove the Weak alkaline coating. If the article should perchance rust or oxidize slight ly during the handling of the article, this oxidation has no particularly deleterious effect, as a matter of fact, it may be beneficial after the article has been coated with the black coating referred 'to above. This auxiliary coating.

whether it be ferrosoferric oxide or any other coating accomplishing the same result. may be referred to as an activating reagent, which may be a chemical or a chemical compound, reacting with the saturated solution to cause the metallic salts therein to be deposited upon the body more rapidly than they would without the presence of this activating reagent.

The application of protective coating Metallic salts may be deposited from a weak acid solution saturated with such metallic salts. For the purpose of illustration, a weak solution of phosphoric acid may be saturated with metallic phosphates such as iron phosphate, manganese phosphate, copper phosphate, manganese dihydrogen phosphate, and zinc phosphate, and possibly other phosphates. This may have an acce1 erator added thereto, such as copper nitrate. Due to the fact that the acid solution is saturated with the metallic phosphates, any neutralization of the acid solution will result in supersaturation when some of the metallic phosphates will precipitate from the acid solution. By the 01d process, this neutralization was accomplished by the acid uniting with the iron on the surface of the article, causing a thin film of acid adjacent to the ferrous body to be neutralized, thereby precipitating the metallic salts from the neutralized film upon the body. If this process were permitted to continue,

a ,metallic phosphate coating would eventually be deposited upon the surface of the article, completely insulating the article from the saturated acid solution, at which time the process would stop. It would probably take an hour or thereabouts, depending entirely upon conditions.

By the use of an accelerating reagent upon the surface of the article, the phosphoric acid is neutralized far more rapidly, thereby precipitating the metallic phosphates upon the surface at a far more rapid rate of speed. By utilizing the black coating described above, which is believed to be ferrosoferric oxide, it is possible to precipitate a very satisfactory coating in from ten to fifteen seconds.

As a plausible explanation, it is believed that the weak phosphoric acid rapidly penetrates the black coating, so as to become neutralized, thereby precipitating the metallic phosphates upon the surface of the article. As a matter of fact, ten to fifteen seconds produces a very satisfactory metallic phosphate coating. which coating has been found to be far superior in withstanding salt sprays than have phosphate coatings precipitated upon the article by subjecting the cleansed article without-a black coating to the influence of a weak solution of phosphoric acid saturated with metallic phosphates. It may be that the black coating is spongy or porous, so that'the weak solution of phosphoric acid saturated with metallic phosphates fiow into the interstices, so that at first, the coating may consist of a combination of the black coating and the metallic phosphates deposited therein. If the process is permitted to continue, it is believed that eventually the black coating is replaced entirely by the metallic phosphates.

The explanations made above as to the theories and chemical reactions that may take place have been set forth merely as plausible explanations, without any limitation and without influencing the validity and the scope of the patent in the event the theories and explanations should be erroneous.

Having thus described my invention, I claim:

The method of applying a protective coating to a ferrous body including the following steps: dipping the body to be coated into a bath of fused sodium hydroxide provided with an electrode, passing an electric current from the body functioning as a cathode to the electrode functioning as an anode so as to remove the iron oxides and other foreign matter from the surface of the body to be treated, reversing the polarity of the current causing an oxide coating to be formed upon the surface of the body, removing the body from the sodium hydroxide, rinsing the body and dipping it into a weak phosphoric acid solution saturated with metallic phosphates of the group consisting of iron phosphate, manganese phosphate, zinc phosphate and copper phosphate having added thereto copper nitrate, the oxide coating on the surface of the body neutralizing the film of phosphoric acid in contact with the body so as to precipitate the metallic phosphates upon the surface of the body to thereby form a protective coating.

ELDEN D. BAYLEY, JR. 

